The present invention relates to a scanning optical system employed in a scanning optical device such as a color laser beam printer.
A scanning optical device is provided with a scanning optical system, a developing unit, a fusing unit, a sheet feeding mechanism or the like.
In the scanning optical system, a laser beam emitted from a laser source is deflected by a polygon mirror and forms a beam spot on an object surface such as a photoconductive drum through an imaging optical system. The beam spot formed on the object surface moves (i.e., scans) on the object surface in a predetermined scanning direction as the polygon mirror rotates.
In this specification, a scanning direction of the beam spot on the object surface is referred to as a xe2x80x9cmain scanning directionxe2x80x9d and a direction perpendicular to the main scanning direction on the object surface is referred to as an xe2x80x9cauxiliary scanning directionxe2x80x9d. Shapes and orientations of powers of respective optical elements will be defined on the basis of these scanning directions.
The laser beam is modulated by an input signal to form a latent image onto the object surface and the latent image is developed by the developing unit with toner. The toner is transferred to a sheet of paper and fused in place by the fusing unit.
In a scanning optical system, a width of a scanning range on the object surface may be out of order due to processing error of the optical elements or misalignment thereof. The error of the width of the scanning range causes size error of an image printed on the paper.
The problem of the size error becomes increasingly serious in a tandem type scanning optical device such as a color laser beam printer that has a plurality of scanning optical systems. A tandem type scanning optical device for a color laser beam printer is provided with four laser sources and four photoconductive drums that correspond to colors Y (yellow), M (magenta), C (cyan) and K (black), respectively. Four light beams are deflected by the polygon mirror and converged through the imaging optical systems to form scanning lines on the respective photoconductive drums. In the tandem type scanning optical system, the size error in at least one of the scanning optical systems causes mismatching of colors, which is a critical defect for the color laser beam printer.
It is therefore an object of the invention to provide an improved scanning optical system that is capable of correcting the size error of the printed image.
For the above object, according to the present invention, there is provided a scanning optical system, including a laser source for emitting a laser beam, a scanning deflector that deflects the laser beam, an imaging optical system that converges the scanning laser beam onto an object surface, first and second mirrors that bend the optical path of the scanning laser beam, the first and second mirrors being movable to adjust the optical path length between the deflector and the object surface for changing a width of the scanning range on the object surface.
With this construction, since the optical path length is adjusted by moving the first and second mirror, which changes the width of the scanning range, correcting the size error of the printed image. Therefore, when the size error is detected, an operator moves the first and second mirrors to correct it. In the case of the tandem type scanning optical device, the operator adjusts the first and second mirrors of the respective scanning optical systems in order to match the widths of the respective scanning ranges to each other.
However, the width of the scanning range cannot be changed when the first and second mirrors are located in an optical path where the scanning beam at any scan angle is parallel to the optical axis of the imaging optical system. Therefore, at least one of the first and second mirrors should be located in an optical path where the scanning beam whose scan angle is not zero is not parallel to the optical axis of the imaging optical system in order to change the width of the scanning range by the movements of the first and second mirrors.
The first and second mirrors may move while keeping the position of the scanning line formed on the object surface in an auxiliary scanning direction. On the other hand, when the movements of the first and second mirror change the position of the scanning line in the auxiliary scanning direction, the timing of the applied signal to the laser source should be shifted.
It is preferable that the optical path between the deflector and the first mirror intersects the optical path between the second mirror and the object surface to reduce the occupied space of the scanning optical system.
The moving amount of the second mirror may be proportional to the moving amount of the first mirror.
Further, the first and second mirrors may be supported so as to be moved as a single-piece. Particularly, the first and second mirrors can be formed as a single-piece.
The imaging optical system may comprise a plurality of lens elements. In such a case, the first and second mirrors may be arranged between the lens elements. The lens element between the second mirror and the object surface may be moved with the movement of the second mirror.